The following co-pending and commonly assigned U.S. patent applications have been filed on the same day as this application. All of these applications relate to and further describe other aspects of the embodiments disclosed in this application and are incorporated in this application by reference in their entirety.
U.S. patent application Ser. No. 09/747,597, xe2x80x9cAUTOMATIC BRIGHTNESS CONTROL SYSTEM AND METHOD FOR A DISPLAY DEVICE USING A LOGARITHMIC SENSOR,xe2x80x9d filed on Dec. 22, 2000, and
U.S. patent application Ser. No. 09/748,615, xe2x80x9cVARIABLE RESOLUTION CONTROL SYSTEM AND METHOD FOR A DISPLAY DEVICE,xe2x80x9d filed on Dec. 22, 2000.
This invention generally relates to display devices. More particularly, this invention relates to display devices having offset error reduction for brightness resolution control.
Display devices are used in a variety of consumer and industrial products to display data, charts, graphs, messages, other images, information, and the like. Backlight display devices, which may be backlit or frontlit, have a backlight positioned to provide light for a display panel. Emissive display devices have pixels that are the emissive light source. In emissive displays, the pixel light source may be CRT phosphor, FED phosphor, a light emitting diode (LED), an organic diode, an electroluminescent, or any emissive display technology. In backlight display devices, the backlight may be a fluorescent tube, an electro-luminescent device, LED, a gaseous discharge lamp, a plasma panel, and the like. The display panel may be a passive or active matrix liquid crystal display (LCD). The backlight and display panel are connected to control circuitry, which is connected to a voltage supply. The display device may be separate or incorporated with other components, such as a dashboard in an automobile or other vehicle, a portable electronic device, and the like.
Generally, a display device controls brightness in relation to the environment of the display device and user preferences. In some applications, the brightness may remain at an essentially fixed level for an extended time period. In other applications, the brightness is adjusted frequently because of changes in the environment, user preferences, and similar factors. The control circuitry may automatically adjust the brightness. A user may further adjust or manually set the brightness through a user interface, such as a knob, switch, keypad, touch screen, remote device, or the like.
To change or adjust the brightness, the control circuitry receives an input signal indicating a user preference, an environmental condition, or the like. The control circuitry selects a luminance value corresponding to the input signal. The luminance value is converted into an analog control signal or an output voltage. The control circuitry provides the analog control signal to the backlight, the display panel, or both. The control circuitry may modify or further adjust the analog control signal and may combine the analog control signal with other inputs to operate the display device at the desired brightness.
The control circuitry typically has a single digital-to-analog converter (DAC) or PWM plus a filter to convert the luminance into the analog control signal. A higher resolution DAC may be used to provide sufficient adjustment resolution for lower levels, the dynamic range, and an exponential output signal. A typical DAC for brightness resolution control may have 12 bits for use in a dynamic range of about 0.5 nits through about 400 nits.
During the digital to analog conversion, the DAC may introduce offset errors into the analog control signal or output voltage. Offset errors are inherent to DACs and may result from the digitizing process and other factors. Offset errors generally are constant errors over essentially an entire dynamic range. Other DAC errors such as quantization errors and linearity errors may result. For digital processing, signal values may be rounded or truncated to form an integer. A quantization error may result when the responsive analog control signal provides a brightness level different from the brightness level corresponding to the selected luminance value. As brightness resolution increases, more quantization errors may result due to the increase in brightness adjustment steps and other factors. Additionally, as the brightness level decreases, the offset error and quantization error increases the error of the desired output brightness.
At lower display luminance levels, there may be more offset errors and more noticeable offset errors. While the digital data input into the DAC typically has a linear progression, the analog control signal from the DAC has constant ratio steps or an exponential progression for a user to perceive the brightness adjustments. Brightness adjustments need constant ratio steps which results in the need for variable resolution control because of how a human eye perceives changes in brightness. The human vision system perceives changes in brightness non-linearly and logarithmically. A user perceives a brightness change from about 10 nits to about 12 nits as essentially equal to a brightness change from about 100 nits to about 120 nits. As the brightness level decreases, more brightness control resolution is needed to accurately provide the brightness step changes that are perceived as uniform by a user. This exponential progression may make offset and quantization errors more noticeable to a user at lower luminance levels. A brightness offset and quantization error of 1 nit is about one percent of a brightness level equal to 100 nits. The same brightness error is about 10 percent of a brightness level equal to 10 nits. As a result, the acceptable amount of brightness offset error decreases as the luminance or brightness level decreases.
Offset errors generally are unacceptable, especially at lower luminance values. A higher resolution DAC may reduce the offset errors, but increase the cost of the display device. A higher resolution DAC may reduce the quantization errors, but increase the cost of the display device and generally does not significantly reduce the offset error because virtually all DACs have offset error. Other approaches include correcting the offset error on a per unit basis or using a complex feedback system that requires a precise digital-to-analog converter with corresponding software to provide the offset error correction. These approaches are difficult to implement and may increase the cost of the display device.
This invention provides a brightness offset error reduction system for display devices. The brightness offset error reduction system may divide the output voltage from digital-to-analog (DAC) circuitry used to control the brightness of the display device. This division of the output voltage may be used to reduce brightness offset errors and may be used to increase the brightness resolution at low luminance levels, such as nighttime applications. The brightness offset error reduction system may be used in automotive and similar applications where the maximum nighttime brightness is a divided ratio of the maximum daytime brightness.
In one aspect, a display device with a brightness offset error reduction system has a lighted display, digital-to-analog (DAC) circuitry, and voltage divider circuitry. The voltage divider circuitry is operatively connected to receive an output voltage from digital-to-analog converter (DAC) circuitry. The voltage divider circuitry provides a fractional portion of the output voltage as a divided output voltage to the lighted display.
In another aspect, a brightness offset error reduction system for a display device has digital-to-analog converter (DAC) circuitry and voltage divider circuitry. The voltage divider circuitry has a switching mechanism and is operatively connected to receive an output voltage from the DAC circuitry. The voltage divider circuitry provides a divided output voltage when the switching mechanism is engaged.
In a method for reducing the brightness offset error, a luminance value is converted into an output voltage. A determination is made whether the output voltage is to be divided. A fractional portion of the output voltage is provided when the output voltage is to be divided.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.